Children in schools as “super-spreaders” of Sars-Cov-2: myth
or truth?
Having established during the first COVID-19 outbreak that protecting
school-aged children from Sars-Cov-2 infection was largely unnecessary,
the rationale for restricting in-presence school attendance lay entirely
in its potential epidemiological benefits in protecting the adults in
their households and the educational staff in their schools. Keeping
students at home directly reduces in-school and public transportation
social contacts for all of pupils, teachers and school staff, thus
limiting the mixing of households and horizontal viral transmission, as
well as limiting the interactions between children, who have been
repeatedly labelled as potential “super-spreaders” of infectious
diseases on account of their increased crowding compared to adults. It
also holds secondary pandemic mitigation potential signalling the
severity of the public health emergency and keeping caregivers at home,
albeit harder to quantify.
The questions now arise: in the case of the COVID-19 pandemic and based
on what is currently known, is the idea of children in the
school-setting acting as “super-spreaders” plausible? Is it reasonable
to suspect that the second wave of the Sars-Cov-2 virus spread in the
WHO European region has been driven also by school re-opening to
in-presence learning for the 2020-2021 academic year? Two key unknowns
should be investigated to understand Sars-CoV-2 infectiousness in
paediatric age. First, what is the susceptibility of children and
adolescents to the infection, and are there differences in attack rates,
length of the infection and viral shedding between the two age groups?
Second, depending on their susceptibility to the infection and their
opportunity for onward transmission, how likely are children and
adolescents to become the index case of COVID-19 clusters, respectively?
With regards to the first key unknown, preliminary evidence collected
during the first COVID-19 outbreak elucidated that adults appeared to be
at a higher chance of becoming infected with Sars-CoV-2 on exposure
compared to younger age groups. This starting point for interpreting the
infectiousness of Sars-CoV-2 in children comes from a large metanalysis
published at the end of September 2020 [13] that included 32
studies. Evolving evidence strengthens these preliminary ones: children
and adolescents are not spared from the infection, but they are a
repeatedly underrepresented population of COVID-19 cases. This is
logically true for the very few seroprevalence studies conducted on
symptomatic cohorts [14-15], susceptible to an inclusion bias due to
the high prevalence of subclinical infection in the younger populations.
More significantly, this is also true for the few large seroprevalence
studies on random cohorts selected independently of symptoms, in
countries where children were continuously exposed to the infection
because of continued in-presence learning or in which the exposure
probabilities were comparable across generations [16-19].
Unfortunately, this epidemiological fact is not established
biologically: when it comes to viral load during the infection and
duration of viral shedding, especially in symptomatic cases, there seem
to be fewer differences between adults and children of all ages
[20]. Less has been confirmed for asymptomatic cases: the full
demonstration of a significantly shortened or lessened infectiousness of
asymptomatic cases would indisputably strengthen all positions favouring
school opening, giving a reproducible explanation for the observed lower
susceptibility to Sars-CoV-2 of the younger demographic groups.
The consistency of the epidemiological evidence alone encourages the
claim that children should not be a primary target population of
COVID-19 pandemic mitigation strategies [21] and has been used as
sufficient evidence to disclaim the role of children as super-spreaders
[22], but it does not disregard the risk that children are
susceptible to the infection and may hence transmit it both in their
classroom and within their household. As a very recently published
modelling study [23] discloses, when it comes to the dangers of
seeding COVID-19 cases across a large community through extensively
undetected school clusters, even an extremely low number of child index
cases might eventually amplify and result in a sizeable community spread
with increased overall incidence and mortality. Confirming this concern
are two large early studies coming from Asia [24,25]. A number of
other studies strongly contradict this evidence, 57 of which were
included in an international metanalysis on household Sars-CoV-2
transmission clusters during the first pandemic wave [26], finding
that children had lower secondary attack rates than adults and that they
infrequently represented the index case. This data was partly
re-confirmed for the second wave in a large UK study [27].